Automatic numerical controlled machines are used to perform a variety of machining operations, such as milling, boring, drilling, reaming, honing, and threading. For each such operation, the exact location, the amount of material to be removed, the finished dimension, and the diameter and depth of holes to be machined, are predetermined and translated into a numerical code which is stored on magnetic tape, perforated tape, cards or the like. The machine is thus "numerically controlled" to perform many machining operations automatically without the requirement for human operators in constant attendance. Such a sequence of operations normally requires the use of a variety of tools, and so the selection of the specific tool needed for each operation and its insertion in the machine spindle is accomplished by numeric control of an automatic tool changer. The automatic tool changer selects a specific tool from a plurality of different types and sizes of tools in a storage magazine and transfers the selected tool to the spindle of the machine while simultaneously removing the previously used tool from the spindle and depositing it in the storage magazine in the position from which the newly selected tool was removed. Such a tool changing operation requires significantly less time to effect a tool change than a system which removes a tool from the spindle and searches for the position in the storage magazine from which the tool was removed and then searches the magazine for the next tool and subsequently inserts the tool in the spindle. This latter operation employs "position coding" of the magazine, whereas the instant operation employs "tool coding" which allows an old tool to be placed in the storage position from which the new tool has just been removed. It is conceivable that after a number of machine sequential operations a specific tool may have occupied every position in the tool storage magazine, without ever having impaired the machine's ability to find that tool when it is next needed.
One of the tool coding techniques currently in use employs a plurality of axially spaced lands and grooves, which may be binary coded, on each of the tool holders for facilitating the selection of a particular tool from a plurality of different types and sizes of tools. The lands and grooves are provided by a plurality of rings which are slipped over the outside diameter of the tool holder. Usually two sizes of rings are utilized, each having the same inside diameter to slidably fit the tool holder, each having the same thickness, and differing only in outside diameter, the smaller diameter ring serving as the groove, and the larger diameter ring serving as the land. These lands and grooves function as the two numerals "0" and "1" in the binary numbering system. Various combinations of grooves and lands produce binary numbers 1, 2, 4, 8 and 16 depending on their relative position along the tool holder and, therefore, require the shop man to be familiar with the binary numbering system or to use a code card to compare against the tool in order to know the cumulative value of the binary digits comprising the coded number. This has been the cause for confusion and error among machine shop personnel, both in setting up such tools and in subsequent indentification of the tool in usage. Additionally, there has been some difficulty experienced by personnel in the mounting of tools in their tool holders to the precise dimensional requirements of a fully automated numerical control system.